t7704: demonstrate failure with once-cruft objects above the geometric split

t7704: demonstrate failure with once-cruft objects above the geometric split

Add a test demonstrating a case where geometric repacking fails to
produce a pack with full object closure, thus making it impossible to
write a reachability bitmap.

Mark the test with 'test_expect_failure' for now. The subsequent commit
will explain the precise failure mode, and implement a fix.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>